A novel excitation scheme for an ocean wave energy converter

This dissertation presents a design for a novel water intake mechanism for a buoy-type ocean wave energy converter (WEC). These renewable energy-harvesting devices float in the open sea and are set into vertical oscillatory motion by incident ocean waves, extracting energy from the relative motion of two or more component bodies. Energy extraction in a WEC can be performed by a variety of power takeoff systems (PTO), such as hydraulic, overhead turbines, or linear magnet generators. The work presented in this dissertation is concerned with improving the power harnessing capabilities of buoy-type WECs. To this end, we propose a novel mass modulation scheme and present designs for the associated water intake mechanism. The water intake mechanism traps and releases surrounding water as needed, thereby leading to a variation of the system's mass. The mass modulation is designed to improve the power harnessing potential of a WEC by varying the system mass at a rate of twice the frequency of the incident ocean waves. To investigate the feasibility of the mass modulation scheme, a simple numerical model for a WEC equipped with the water intake mechanism is proposed and analyzed. Of particular interest is the relationship between mass modulation and energy harvesting, as well as the stability implications for the WEC of such a mass variation. The motions of the system have been studied in response to a spectrum of harmonic excitation and the results were catalogued. Numerical simulations have also been used to demonstrate that when applied correctly, mass modulation can lead to a significant increase in system response and power harnessing potential of a WEC. A scale prototype of a WEC with the water intake mechanism has been constructed and tested in a wave tank to prove the concept of the proposed mass modulation scheme. The results presented in this dissertation show that a WEC fitted with the water intake mechanism exhibits a higher vertical velocity response, compared to a non-mass modulated WEC. In addition, the relative velocity between the WEC and a secondary float can also be increased using the mass modulation. These velocity increases can lead to improvements in the power harnessing potential of the WEC.